Seoul: After quitting his job, Gil-gu (Ahn Bo-hyun) spends his days aimlessly, often trying his luck at crane machines and loitering at a convenience store late at night.
According to Yonhap News Agency, his mundane routine is disrupted when Seon-ji (Im Yoon-ah) moves into the apartment below his, and he is immediately smitten by her. However, he soon discovers there is more to Seon-ji than meets the eye. While she is a polite and well-mannered baker during the day, she transforms into a completely different person in the wee hours, complete with heavy makeup, vivid-colored nails, and strange, wild behavior.
One night, Gil-gu encounters Seon-ji's father, Jang-su (Sung Dong-il), who seems to shadow her during these nightly episodes. Having earned Jang-su's trust, Gil-gu is asked to watch over Seon-ji for a few hours daily.
Gil-gu readily accepts the offer, embarking on an unexpected wild ride, through which he begins to unravel the truth about the "demon" that seems to possess her each night.